The present invention relates to a scanning optical system employed in a laser scanning unit for a laser printer and the like.
In scanning optical systems, a light beam emitted from a laser light source is deflected by a deflector such as a polygon mirror and is converged on a surface to be scanned such as a photoconductive surface of a photoconductive drum or the like as a beam spot, through a scanning lens such as an f.theta. lens. As the polygon mirror rotates, the beam spot on the photoconductive drum scans the photoconductive drum in a main scanning direction. By modulating the laser beam as the polygon mirror rotates, an electrostatic latent image is formed on the surface to be scanned.
In general, it is considered that a laser beam used as the light source of a scanning optical system has a single wavelength. Therefore, conventional scanning optical systems are designed with respect a predetermined wavelength, and correction of chromatic aberration due to various wavelengths was not taken into consideration.
However, laser diodes, which are used as light sources, may have individual differences in characteristics, and may emit laser beams having different wavelengths. If the wavelength of the emitted laser beam is different from the designed wavelength, drawing characteristics are changed due to the lateral chromatic aberration of a scanning optical system. Further, if a scanning optical system is a multi-beam type scanning optical system utilizing a plurality of laser diodes, drawing characteristics are changed for each scanning line due to the variation of the wavelength among the plurality of laser diodes.
It may be possible to correct the chromatic aberration using positive lenses and negative lenses having different dispersion, as is utilized in a camera lens system. In such case, however, there is a problem that the number of lenses constituting the scanning optical system is increased. In addition, a scanning optical system should be newly designed, and when a molded lens is used, a mold must be newly prepared.